The spectacular growth of video, voice, and data communications over the Internet and the equally rapid upsurge of mobile telephony are spurring wideband multiple access research to provide very high data rate transmission in wireless communication systems. Direct sequence code division multiple access (DS-CDMA) is one of the effective wireless access technologies for supporting high system capacity, including variable and high data rate transmission services, and DS-CDMA has been adopted in 3rd generation wireless communication systems. See 3GPP Specification Home Page: http://www.3gpp.org/specs/specs.htm. Owing to its one-cell frequency reuse, DS-CDMA is advantageous in bandwidth efficiency compared to time-division multiple access (TDMA) in a multi-cell environment. However, conventional single-carrier DS-CDMA systems are associated with multiple access interfaces (MAI), which limits the maximum data rate and the system capacity that can be supported for available bandwidth, especially in hot spot environments with isolated or fewer co-existing cells not taking full advantage of one-cell frequency reuse.
Combining orthogonal frequency division multiplexing (OFDM) and CDMA, which is called multicarrier CDMA (MC-CDMA), can improve channel capacity utilization under multipath interference and frequency selective fading reception by multipath delay suppression and diversity gain. See S. Hara and R. Prasad, “Overview of Multicarrier CDMA”, IEEE Comm. Mag., pp 126-133, December 1997. MC-CDMA has been proposed as a candidate for future wireless communication systems. Even though this technology can support high data rates and multiple users, it suffers from high peak-to-average power ratio (PAPR) problems and high sensitivity to frequency offset, RF noise, and channel estimation errors. These limit the applicability of MC-CDMA in practical wireless environments.
Recently, interleaved frequency-division multiple-access (IFDMA) has been introduced as a new wideband spread-spectrum multiple access scheme for both downlink and uplink mobile communications. See M. Schnel and I. De Broeck, “A promising new wideband multiple-access scheme for future mobile communication systems”, European Trans. on Telecomm., Vol. 10, No. 4, pp. 417-427, July-August, 1999; and M. Schnel and I. De Broeck, “Interleaved FDMA: Equalization and coded performance in mobile radio applications”, IEEE ICC '99, pp. 1939-1944, June 1999. The basic idea behind IFDMA is to combine spread-spectrum multicarrier transmission (spread signal bandwidth through signal compression and repetition) with frequency-division multiple-access (FDMA) to avoid MAI and be capable of achieving frequency diversity. Since IFDMA is an orthogonal multiple access scheme, there is, theoretically, no MAI in IFDMA systems. Compared to MC-CDMA, CDMA, and TDMA, IFDMA shows several additional advantages including continuous transmission and constant envelope (low PAPR).
More recently, variable spreading and chip repetition factor CDMA (VSCRF-CDMA) has been proposed as a promising candidate for uplink broadband wireless access. See Y. Goto, T. Kawamura, H. Atarashi, and M. Sawahashi, “Variable spreading and chip repetition factor (VSCRF)-CDMA in reverse link for broadband wireless access”, IEEE PIMRC '03, September 2003. It combines IFDMA with CDMA to take advantages of both technologies. Specifically, VSCRF-CDMA uses two-layer spreading, i.e. code domain spreading as in CDMA and chip-compression-and-repetition (CCR) spreading as in IFDMA. In a multi-cell cellular system, VSCRF-CDMA sets the chip-repetition factor (CRF), which represents the number of users who can be supported by CCR spreading, to one, and it works just like a DS-CDMA system to realize one-cell frequency reuse. In a hot-spot system, CRF is set to more than one and total spreading factor (TSF) is the multiplication of CRF with the code-domain spreading factor (CSF). Orthogonality between users is maintained, and hence MAI is minimized by the introduced CCR spreading in the hot-spot systems as in IFDMA. Furthermore, seamless handoff can be realized for users going from one to another between the cellular system and the hot-spot system, due to the same air interface being deployed.
Other-cell interference (OCI) and MAI issues are crucial in wireless communication systems for spectrum utilization efficiency and system capacity. Although IFDMA is capable of eliminating MAI using CCR spreading, it cannot handle interference from other cells in the same carrier frequency. This is because CCR spreading has only limited interference suppression capability for the same carrier frequency, unlike code spreading gain or channel coding gain, although IFDMA realizes orthogonality between users at different carrier frequencies as in FDMA. Hence one-cell frequency reuse cannot be realized in IFDMA systems.
IFDMA-based VSCRF-CDMA mitigates this problem by using CDMA spreading in cellular systems and code spreading coupled with CCR spreading in hot-spot systems. For cellular systems, other-cell interference (OCI) is minimized through using CDMA code spreading gains and scrambling codes for randomizing interference. In isolated-cell or hot-spot environments, CDMA spreading is not performed because of substantially lower OCI while MAI is reduced by IFDMA-based CCR in each cell. Since CCR spreading is more capable of maintaining orthogonality between different users than code spreading, CCR spreading usually has a higher priority than code spreading to combat MAI. The code spreading in VSCRF-CDMA is a frequency domain spreading, and the orthogonality between different spreading codes (for both scrambling and channelization codes) is subject to frequency selectivity fading or delay spread of the channel. In a frequency selective wireless channel, CSF has to be small enough to maintain code orthogonality after passing through the channel. Therefore, VSCRF-CDMA has limited OCI suppression capability owing to small CSF.
Another cost of using IFDMA and VSCRF-CDMA to achieve the aforementioned advantages over other multiple access techniques is to allow inter-symbol interference (ISI). To reduce the ISI effect significantly, an optimal equalizer based on maximum-likelihood sequence estimation (MLSE) has to be employed at the receiver. The MLSE equalizer has higher complexity and it increases exponentially along with the Q factor, which is the number of symbols per block (unit block for performing repetition) to be transmitted in IFDMA or the number of chips per block (unit block for performing repetition after CDMA spreading) in VSCRF-CDMA.
In general, IFDMA is incapable of handling OCI and hence it is difficult for IFDMA systems to realize one-cell frequency reuse. VSCRF-CDMA mitigates this problem by combining IFDMA with CDMA. However, it has limited OCI suppression capability when using only frequency domain spreading with a small spreading factor. Furthermore, both IFDMA and VSCRF-CDMA require the complex MLSE detection at the receiver. Therefore, there is a need to combat OCI and MAI simultaneously and more efficiently, and to reduce receiver complexity substantially.